In order that the instant invention may be clearly understood, it is useful to review the prior art relating to photochemical isotope separation. U.S. Pat. No. 2,713,025 and British Pat. No. 1,237,474 are good examples of processes for the photochemical separation of the isotopes of mercury. The first requirement for a photochemical isotope separation is that one finds conditions such that atoms or molecules of one isotope of a given element absorb light more strongly than do atoms or molecules of another isotope of said element. Mercury is a volatile metal and readily forms a vapor of atoms. Said atoms absorb ultraviolet light at 2537 A. The absorption line of Hg.sup.202 is displaced by about 0.01 A with respect to the absorption line of Hg.sup.200. Since the absorption lines are extremely narrow, one may by use of a light in a critically narrow wavelength region excite either Hg.sup.200 or Hg.sup.202.
The second requirement for a photochemical isotope separation is that those atoms or molecules which are excited by light undergo some process which the atoms or molecules which have not been excited do not undergo, or at least do not undergo as rapidly. A quantum of 2537 A ultraviolet light imparts an excitation of 112.7 Kcal/mole to the mercury atom which absorbs it. The number of mercury atoms which at room temperature are thermally excited to this energy is vanishingly small, hence the atoms excited by light are not diluted by atoms excited by thermal means. Atoms of this high excitation readily undergo reactions with H.sub.2 O (as taught in the U.S. patent) or with O.sub.2, HCl or butadiene (as taught in the British patent), said reactions not occurring at room temperature with unexcited mercury.
Uranium, however, is a highly refractory metal, boiling only at extremely high temperatures. Thus, use of the above-described process with uranium atoms instead of mercury involves obvious difficulties. The most volatile form of uranium is UF.sub.6. U.sup.235 F.sub.6 and U.sup.238 F.sub.6 both absorb ultraviolet light and do so to exactly the same extent at all wavelengths in the UV; hence, UV excitation of UF.sub.6 does not satisfy the first requirement of photochemical isotope separation. However, UF.sub.6 will also absorb infrared light in the region around 626 cm.sup.-.sup.1 (the V.sub.3 band) and 189 cm.sup.-.sup.1 (the V.sub.4 band. Both the V.sub.3 and V.sub.4 bands of U.sup.235 F.sub.6 are shifted slightly toward higher energy with respect to the V.sub.3 and V.sub.4 bands of U.sup.238 F.sub.6 respectively, but the size of these shifts is small compared to the width of the bands; in other words, the infrared absorption spectra of U.sup.238 F.sub.6 and U.sup.235 F.sub.6 do not exactly coincide, but they overlap at all wavelengths so that if one isotope absorbs light, so, to a substantial degree, will the other. Hence, the infrared excitation of UF.sub.6 by absorption of a single IR photon is a process of limited isotopic selectivity.
The second requirement for isotope separation is also a matter of some difficulty for UF.sub.6. UF.sub.6 molecules which are excited by IR light are no different from molecules which have been excited to the same energy level thermally. Most processes the photoexcited molecules will undergo, those molecules which are thermally excited to the same energy level will also undergo. This dilution of the photoexcited molecules with thermally excited molecules will further decrease the isotopic separation factor.
The instant invention is a three-step process, the first step being that the UF.sub.6 molecules to be isotopically separated are irradiated with a powerful infrared laser for a time of less than 10.sup.-.sup.3 seconds under conditions such that at least 0.1% of the U.sup.235 F.sub.6 molecules being irradiated absorb an energy of more than 2000 cm.sup.-.sup.1. This may be done by use of the process of sequential multiple photon absorption, i.e. if UF.sub.6 is irradiated at a power density greater than 10.sup.4 watts per cm.sup.2 per torr pressure of UF.sub.6 in the presence of a second gas, said second gas having a partial pressure of at least 5 times the partial pressure of the UF.sub.6, then the UF.sub.6 may be sequentially excited from the ground vibrational state to the first excited vibrational state to the second excited vibrational state to the third excited vibrational state to the fourth excited vibrational state, etc., the isotopic selectivity of each excitation step being compounded as the sequential excitation proceeds.
The second step of the instant invention is the reaction of said excited UF.sub.6 with a gaseous reagent to produce a product which is recovered in the third step by means known in the art. The ratio of said gaseous reagent to UF.sub.6 must be at least 0.1. The total time in which the UF.sub.6 is in contact with said gaseous reactant, both before irradiation by the IR laser and after said irradiation, is less than 10.sup.-.sup.3 seconds. Said gaseous reagent is chosen from the group consisting of atomic chlorine, bromine, and iodine.
The reasons for the above requirements and the preference for atomic chlorine, bromine, and iodine are complex and are related to the problem of the thermal dilution. As indicated above, UF.sub.6 molecules which are excited in an isotopically selective manner by the IR laser will be diluted with UF.sub.6 molecules thermally excited to the same energy levels. It is the teaching of the instant invention that this undesirable dilution effect is to be minimized by rapidly exciting a significant fraction (more than 0.1%) of either the U.sup.235 F.sub.6 or U.sup.238 F.sub.6. Once the photoexcited UF.sub.6 molecules are formed, they will be removed by reaction with the atomic chlorine, bromine, or iodine and they will be removed by deexcitation processes. The thermally excited UF.sub.6 molecules will also be removed by reaction and deexcitation but new thermally excited UF.sub.6 molecules will be continually formed from unexcited UF.sub.6 molecules. Thus, after irradiation by the IR laser the ratio of photoexcited UF.sub.6 molecules to thermally excited UF.sub.6 molecules will continually decrease and the undesirable dilution effect continually increase. Under most conditions, the lifetime of the photoexcited UF.sub.6 will be 10.sup.-.sup.3 seconds or less and it is the teaching of the instant invention that both the irradiation time and contact time be less than 10.sup.-.sup.3 seconds in order to minimize the dilution effect.
Since this very limited time is available for reaction between the photoexcited UF.sub.6 and the gaseous reagent, it is necessary to use said gaseous reagent in considerable excess over the photoexcited UF.sub.6 in order to achieve an acceptably efficient recovery of the latter. Hence, the instant invention teaches that the ratio of gaseous reagent to total UF.sub.6 must be at least 0.1.
The use of atomic chlorine, bromine, or iodine as the reagent with which the photoexcited UF.sub.6 reacts is advantageous for three reasons. First, there is a general advantage of atomic over molecular reagents. It is well known that when a vibrationally excited molecule collides with an unexcited molecule the excitation may be transferred leaving the former molecule unexcited and the latter molecule excited. This process is called V--V transfer and it can be an extremely efficient process. If one attempted to react the photoexcited UF.sub.6 with a molecular reagent, V--V transfer could result in the rapid loss of the photoexcited UF.sub.6. The use of an atomic reagent which cannot undergo V--V transfer avoids this danger.
Second, within the group of atomic reagents atomic chlorine, bromine, and iodine have the advantage of being readily generated in situ by photolysis, pyrolysis and other means known in the art.
Third, atomic chlorine, bromine, and iodine are well known to undergo a rapid recombination to form molecular halogens. Thus, they are self scavenging reagents, i.e. they may be used in excess over that which will react with the photoexcited UF.sub.6 and the unused excess will react with itself to form the relatively inert molecular halogen. By suitable adjustment of the reaction conditions, generation and substantial removal of atomic chlorine, bromine, or iodine may be achieved within the required contact time of 10.sup.-.sup.3 seconds.
From the above description, the instant invention is readily distinguished from the prior art. Thus, U.S. Pat. No. 3,443,087 teaches the separation of U.sup.235 F.sub.6 from U.sup.238 F.sub.6 by selectively exciting one of them with an infrared laser then ionizing said excited molecules with ultraviolet light and recovering the ions by means of electric and/or magnetic fields or chemical reactions. In a review entitled "Photochemical Isotope Separation As Applied to Uranium" (Union Carbide Corporation Nuclear Division, Oak Ridge Gaseous Diffusion Plant, Mar. 15, 1972, K-L-3054, Revision 1, page 29), Farrar and Smith discuss the above-mentioned patent and comment unfavorably on the practicality of the proposed second step of photoionization. As an alternative, they suggest photodissociation.
British Pat. No. 1,284,620, German Pat. No. 1,959,767 and German Pat. No. 2,150,232 teach the use of infrared radiation to selectively excite molecules which then undergo a chemical reaction which the unexcited molecules undergo more slowly. Only one example of such a reaction is given, the thermal decomposition of U(BH.sub.4 ).sub.4.
In all the above references the energy given the molecules in the photoexcitation step is explicitly taught to be that of one IR photon, which for UF.sub.6 is less than the excitation of at least 2000 cm.sup.-.sup.1 taught in the instant invention. None of the above references teach, show, or suggest the advantage of exciting a substantial fraction of either the U.sup.235 F.sub.6 or the U.sup.238 F.sub.6 and thereby reducing thermal dilution, nor do they teach, show or suggest the need for very short irradiation time and very short contact time, nor do they teach, show, or suggest the need to use an excess of gaseous reagent to efficiently recover the photoexcited UF.sub.6, nor do they teach, show, or suggest the advantages of using atomic chlorine, bromine, or iodine as the said gaseous reagent.